1. Field of Invention
This invention relates to metal plating solutions, and more particularly, to electroless metal plating solutions stabilized with elemental sulfur.
2. Description of the Prior Art
Electroless metal doposition refers to the chemical plating of a metal over an activated surface by chemical or auto-catalytic reduction of metal ions in the absence of an external electric current. Compositions and processes useful for this deposition are in wide commercial use and are described in numerous publications. Examples of electroless deposition plating solutions are described in U.S. Pat. Nos. 2,938,305; 3,011,920; 3,313,224 and 3,361,580, all incorporated herein by reference.
Known electroless metal depositions solutions generally comprise at least four ingredients dissolved in water. They are (1) a source of metal ions, e.g., water soluble salts of a plating metal such as cupric sulfate or nickel chloride, (2) a reducing agent such as formaldehyde for copper plating solutions, a hypophosphite or amine-borane for nickel plating solutions and hydrazine for plating solutions such as palladium, (3) an acid or hydroxide pH adjuster to provide required solution acidity or basicity and (4) a complexing agent for the metal ions sufficient to prevent their precipitation from solution. A large number of suitable complexing agents for electroless metal solutions are described in the above noted patents and also in U.S. Pat. Nos. 2,874,072; 3,075,856 and 3,075,855, also incorporated herein by reference.
It is known in the art that electroless metal plating solutions tend to be unstable and spontaneously decompose, possibly due to the presence of catalytic nuclei in a solution containing both a reducing agent and reducible metal ions.
It is known that this decomposition can be retarded and the life of the plating solution increased by the addition of various solution soluble additives in small concentrations which additives are known in the art as stabilizers. Illustrative examples of said stabilizers are given in U.S. Pat. Nos. 3,310,320; 3,361,580 and 3,436,233 (soluble divalent sulfur compounds); 3,403,035 and 3,310,430 (soluble cyano compounds); and 3,661,597 and 3,457,089 (soluble acetylentic compounds).
In general, these stabilizers are catalytic poisons when used in excess of trace amounts. Therefore, they are typically used in concentrations of a few parts per million parts of solution. Larger amounts can retard the rate of deposition, may even prevent deposition, and frequently adversely effect the ductility and color of the deposit. Such adverse effects have been described in U.S. Pat. No. 3,804,638 and by A. Molenaar et al., Plating 649, (1974). Preferred stabilizers are those which stabilize, but are not catalytic poisons and consequently, do not require strict concentration control nor adversely affect the rate and quality of deposition. For example, mercury compounds, capable of dissociating to yield mercury ions in small concentrations, as described in U.S. Pat. No. 3,663,242, improve bath stability without decreasing the rate of deposition.